The combustion of coal and other fossil fuels during the production of steam or power produces dozens of gaseous oxides, such as NO, NO2, N2O, H2O, HO, O2H, CO, CO2, SO, SO2, etc. which together, with N2 and excess O2, make up the overwhelming majority of the boiler flue gas. Many of these species, such as OH and O2H are highly reactive and are chemically quenched prior to the flue gas exit from the boiler stack. Some of the species, such as CO, CO2, and H2O are highly stable, the vast majority of which will pass unreacted out of the boiler stack. Still other species, such as SO2, NO, and NO2 are moderately reactive. These moderately reactive species are subject to removal from the flue gas by chemical reaction processes, and are also subject to complex variability.
NO and NO2, together referred to as NOx, are gases that are highly toxic to humans. NOx can combine with H2O to form nitric acid in the lungs and other mucosal membranes. In addition, NOx can react with O2 in the lower troposphere to form ozone, also a toxic gas. For these and other reasons, including federal and state legislation, many industrial power plants need to find ways to cost effectively reduce the flue gas NOx to acceptable emission levels.
NOx levels out of the furnace of industrial coal burning power plants vary, depending upon the particular combustion technology, typically from about 100 parts per million (ppm) to greater than one thousand ppm. NOx levels out of the furnace of industrial oil and gas burning power plants vary less, because of the consistency of the fuel sources, from about 100 ppm to 600 ppm. Oil and gas combustion tend to produce lower levels of NOx than does coal combustion because there is little nitrogen found within oil or gas, and because they tend to burn at lower temperature than coal due to their relatively simple molecular structures. Desirable emission levels vary, depending upon many factors including plant location, stack height, furnace NOx levels, and state laws and provisions. Desirable emission levels may be expressed as an absolute number, e.g. less than 50 ppm, or as a percentage reduction from the furnace NOx levels. Desirable NOx emission levels can be as low as 10 ppm.
A number of technologies have been adapted by industrial power plants in order to reduce furnace NOx levels to acceptable emission levels. These include SCR, Selective NonCatalytic Reduction (SNCR), boiler optimization and tuning, the use of Over Fire Air (OFA) and other vertical staging of furnace air introduction techniques, and the use of low NOx burners. Each method has a typical NOx reduction range that tends to correlate well with the cost the market will bear for the device. Of the methods listed, the SCR is capable of removing the greatest quantities of NOx from the flue gas but does so at a very high cost. Design and installation costs for industrial SCRs run into hundreds of millions of U.S. dollars. Annual operational costs, including reducing reagent, maintenance, and catalyst replacement, for an industrial SCR can run into the millions of dollars.
An SCR works by adding a reactive chemical reducing agent into the flue gas in appropriate stoichiometric amounts so that it may react with the NOx molecules and undergo a chemical reaction into harmless byproducts. A catalyst, typically a solid phase metal oxide deposited on a support structure, is used to accelerate the rate of key steps in the chemical reactions. Typically, a catalyst will drop the energy of activation for a reaction by transiently binding with one or more of the reagents. One of the most commonly used catalysts found in SCR is di-Vanadium Pentoxide (V2O5). In addition to the active catalytic molecule, SCRs may contain molecules whose purpose is to impact other important reactions, such as impeding the catalyst poisoning from Na, K, AsO3, Pb, P, Cl, F, etc. The additives vary between vendors and differ according to the SCR design, fuel being burned, and other factors.
Nearly all SCRs use ammonia (NH3) as the reducing reagent. Though there are many reactions that may be used to reduce NOx into harmless byproducts, one of the more common isNH3+NO+¼O2→1½H2O+N2 indicating that ammonia and NO combine in a one to one ratio, and with oxygen in a 4 to 1 ratio, to produce water and nitrogen gas. This and other reaction mechanisms indicate the actual stoichiometric ratio of NOx and reducing reagent that get consumed by the reaction. If the actual flue gas contains an excess of either ammonia or NOx, that excess will leave the SCR, where it can subsequently react or remain in the flue gas until it exits at the stack. Excess ammonia is considered undesirable because it can render trapped fly-ash unsellable due to odor and because it is a very toxic and corrosive gas that should not exit the stack in even small amounts. Excess NOx is considered undesirable because this indicates that the SCR was not operating at its full potential efficiency. Excess NOx or ammonia are referred to as “slip”.
SCR manufacturers have developed numerous methods and devices in an attempt to achieve the tightest stoichiometric matching of injected reducing reagent to furnace NOx, and thereby achieving the highest effectiveness and efficiency of SCR performance. There are several challenges to matching the stoichiometry of the reagent and the NOx.
One challenge to SCR optimization is to adjust the ammonia injection to the continuously varying distribution of NOx that comes out of the furnace. Levels of NOx can vary by a factor of two or more as measured laterally, front to back or left to right, across the duct. The variations can happen rapidly and are a function of both exogenous (outside of the immediate operator or automatic control) and endogenous factors. Factors include load, temperature, coal type, unit load, burner tilts, lateral fuel biases, lateral air biases, LOI, turbulence, coal particle size, vertical fuel bias, and vertical air bias, including OFA.
Rather than grapple with the problem of constantly matching the reagent injection profile to the NOx profile at all points along the two dimensions of lateral traversal across the duct, some SCR designs rely on the use of mixers to generally remove all lateral variations in the boiler NOx profile. Ammonia may then be injected into the mixed flue gas and the combined gases then mixed again, to attempt to ensure the equal distribution of the ammonia. This method has a thermodynamic efficiency cost related to the loss of exergy from the mixing process. The greater the mixing, the greater the thermodynamic efficiency cost. The effectiveness and corresponding design of mixing devices is dependent in part on the flue gas velocity. Since the mixing devices typically are static structures, they are, by design, only optimized for mixing at the design load. As a result, the mixing efficiency will be sub-optimal at various, off-design, times of operation. Areas with less than perfect mixing will result in poor stoichiometry and associated slip of either NOx or ammonia.
Another way to manage the variability of the NOx distribution is to enable the automated and real time manipulation of the reagent injection profile so that it can be adjusted to match the NOx distribution. A drawback to this method is that it requires the manipulation of multiple valves in the ammonia injection grid (AIG). There is an installation expense to configuring multiple valves for actuation, an operational expense to enabling the real time manipulation of multiple AIG valves, and a complication and associated hazard of adding any movable part into an ammonia system. At present, very few SCRs in the U.S. are designed with this degree of on line control built into the AIG.
Another way to manage the variability of the NOx distribution is to enable the manipulation of the reagent injection profile on a non-real time, periodic basis. Most SCRs in the U.S. use this method, first tuning the AIG profile at commissioning of the SCR and then again on an annual basis. The tuning procedure includes identification of the typical NOx distribution patterns from the boiler and subsequent adjustment of the AIG valves to achieve the desired stoichiometry. The AIG valves are then fixed in this position until the next study is performed. There are numerous drawbacks to this methodology. First, the tuning is optimized for the typical, modal (design) load factors of the boiler. At off-design loads the NOx distribution will change and will result in additional NOx slip or ammonia slip. Another drawback to this method is that it cannot compensate for the slow drifts that occur in the NOx distribution at the modal (and other) load over the period of a year. Another drawback to this method is that it cannot compensate for the variations that occur in furnace NOx distribution that result from different operator or automatic controllers that manipulate burner tilts, lateral fuel biases, lateral air biases, LOI, turbulence, coal particle size, vertical fuel bias, and vertical air bias, including OFA. In effect, this method is optimized for operation at the modal NOx distribution of the furnace and is sub-optimal at all other conditions, resulting in additional NOx slip or ammonia slip.
The lateral furnace NOx profile can change as a function of air and fuel introduction parameters because NOx is created largely out of the Oxygen found in the air and Nitrogen found in the fuel. However, NOx profiles and, more importantly, integrated NOx quantities are strongly dependent upon the combustion temperature of the furnace and therefore upon the load of the unit. The very strong temperature dependence of NOx formation is a result of the usually exponential dependence of chemical reaction rates on temperature.
As a result, another challenge to SCR optimization is to adjust the net ammonia injection amount into the flue gas as a function of combustion temperature. Because combustion temperature is expensive to measure continuously and reliably, most SCRs do not use it as the input parameter for injected ammonia, but rather use unit load, a good proxy for combustion temperature, as the input parameter. Typically, design curves are used to represent the total furnace NOx production as a function of unit load. The load based NOx design curves are typically created during the commissioning of the SCR and may be updated periodically over the life of the SCR. The load based NOx curves are typically used by the DCS or other automated control system for the feed forward control of the total amount of injected ammonia. In particular, as unit load goes up, the feed forward control will anticipate the increased production of furnace NOx and will adjust the total ammonia injection levels accordingly. One drawback to feed forward control is that it does not make adjustments to the injected ammonia as a function of any variable except that which is specified in the curve. So, for instance, a large change in furnace NOx production resulting from a change in coal type would not result in a change in total ammonia injection. For this reason, most SCR control systems that deploy a feed forward control loop also deploy a feed back control loop.
The feedback control loop of the SCR is typically designed to measure the NOx slip (NOx level in the stack) and adjust the feed forward specified ammonia injection amount so as to maintain a constant NOx slip or percentage NOx removal. The feedback loop can therefore correct for variations of total furnace NOx productions that differ from the load based design curves.
Another challenge to SCR optimization is to make accommodations for the fact that the catalyst will degrade and will degrade non-isotropically. Non-isotropic degradation of the catalyst leads to certain flow lines through the catalyst bed having greater or lesser integrated activity than the average. As the average integrated activity drops, those flow lines of lesser activity than average can eventually reach a critical level where complete reaction of the NOx and the ammonia no longer occurs, resulting in both NOx and ammonia slip along those flow lines. This situation is particularly difficult to manage because the standard SCR feedback loops will see an increase in NOx slip and will correct for it by increasing the injected ammonia, which will have the unintended affect of increasing ammonia slip.
The catalyst degradation itself can be due to a number of factors. These factors include high flue gas temperatures (which lead to a phase transition of the catalyst); plugging of the catalyst pores and masking of the catalyst surface (with Calcium and Ammonium sulfate salts or with fly ash); and poisoning of the active catalytic sites (with Na, K, AsO3, Pb, P, Cl, F, etc.).
Non-isotropic catalyst degradation can result from a number of sources. One source is the maldistribution of furnace gases resulting from poorly balanced combustion conditions. Since the flue gas in most industrial power plants is subject to laminar flow conditions, maldistributions at the furnace can manifest in the SCR. Another related source of non-isotropic degradation are temperature spikes along certain flow lines resulting from non-isotropic cleaning of the heat transfer surfaces in the back pass. Another source of non-isotropic degradation is from high levels of ammonia injections in parts of the AIG that lead to enhanced ammonium sulfate condensation in the SCR. Yet another source of non-isotropic degradation is from systematic errors in sootblowing of the catalyst bed.
The overall activity level of the catalyst bed can be defined as a number between 1 and 0, where 1 indicates a completely active, usually fresh and clean, surface and where 0 indicates no active surface remaining. Recommendations vary according to manufacturer, but catalyst beds are typically changed out when the activity drops to anywhere between 0.5 and 0.9. Since changing the catalyst is a very expensive proposition, extending the usable life of the catalyst is desirable. Regardless of the activity level of the catalyst, catalyst change or addition is usually required when ammonia slip levels exceed a certain design specification, which is usually in the range of 2-5 ppm.
Industrial SCRs are built with an excess of catalyst. If the challenges to SCR control described above could be resolved, these SCRs could operate at 100% NOx removal efficiency with negligible ammonia slip. Instead, most SCRs operate at about 80-90% removal efficiency. Enhanced control of these challenges is therefore desirable.